Re: AO-40 MA, ALON/ALAT and squint angle ??

Hi Erich,
Thank-you for your detailed reply, from which I've actually learnt something!
I had previously disregarded precession of the orbit, as the satellite
remains pointing at the same point in space... but as you point out, the
reference system which we are using for satellite attitude is defined with
respect to the orbital plane. Therefore, if the satellite keeps pointing
in the same direction, but the reference point changes, then obviously our
expression of attitude must change too. (Well, it's obvious to me now!)
I think Dr Zickwolff's article should be published on the Amsat-DL webpage,
or somewhere else... do you know who owns copyright on this article?
Thanks again,
Chris vk6kch
At 18:21 02/01/2001 +0100, you wrote:
>Hi Chris,
>thank you very much for the good wishes and your prompt answer.
>What I wanted to say (with: ALON/ALAT are time dependend), is the following:
>As you wrote, the z-axis of a spin stabilized satellite is always pointing
>to a fix point in space (the A point), independend from time and independent
>from the satellites position in the orbit and also independent from
>changings of the orbit plane itself. (unless the satellite is forced to
>change its attitude). This (absolute) point can be determined by the
>coordinate system of the sky (sky equator, sky pole), which is a fix system
>in space. On the other hand, AON/ALAT relate to the coordinate system of the
>satellites orbit plane (where longitude 0 is always in the perigee,
>longitude 180 always in the apogee etc.). This system is not fix in space,
>however. The perigee of the momentary orbit ellipse of AO-40 in the space
>(RefPerigee) moves about 0.4 deg/day, RAAN about -0.20 deg/day. This can
>(and needs to) be calculated by the tracking programs on the base of the
>Keplerian data.
>
>This dayly changings are not very big, but they add up. As I wrote before:
>When I altered the reference day of ALON/ALAT 260/2 by 28 days (from Dec.1st
>to Dec. 28th 2000) the results for the squint angel calculated with the same
>Keplerian data and for the same time (January 1st, 2001, 12.00.00 UTC)
>changed from 95° to 90° for my location.
>
>So, if you want to calculate the A point as a fix point in space by given
>ALON/ALAT, you need to know the day, to which these values refer. It goes
>without saying, that the A point has an influence on the squint angle
>calculations.
>
>This all (the mathematic formulas inclusive) is described by Dr.Gerd
>Zickwolff, DL1TV (he died 2 years ago), in the AMSAT DL Journal 3/1990. He
>calculates the A point on the base of given ALON/ALAT and given day and on
>the base of the actual Keplerian data and transfers its coordinates into
>the unchanging (absolute) coordinate system of the sky. From there the
>actual values of ALON/ALAT can be recalculated for any actual time.
> He clearly says, that the day, to which ALON/ALAT refer, must be known. (He
>also suggested, that the sources should not only tell ALON/ALAT as
>coordinates of the orbit plane, but also the absolute coordinates of the A
>point, to simplify squint calculations.)
>
>Vy 73s, Erich, DK1TB
>www.dk1tb.de
>
>----- Original Message -----
>From: "Chris Hill" <CRH_Telnet@bigpond.com>
>To: "Erich Eichmann" <Erich.Eichmann@t-online.de>
>Cc: <amsat-bb@AMSAT.Org>
>Sent: Tuesday, January 02, 2001 1:51 AM
>Subject: Re: [amsat-bb] AO-40 MA, ALON/ALAT and squint angle ??
>
>
>> Hi Erich,
>>
>> Happy New Year to you too! Hope 2001 is full of exciting things (like
>lots
>> of Amateur Radio fun via AO-40, etc!)
>>
>> I think you need to distinguish between ALON/ALAT, and squint angle. They
>> are two separate issues, although squint angle is partly dependent upon
>> what the ALON/ALAT is.
>>
>> For a satellite which is spinning sufficiently fast, the ALON / ALAT will
>> _not_ change on a day to day basis... it will remain fixed. The
>> satellite's attitude can only change as the result of applied forces,
>> either deliberately induced (eg magnetorqueing at perigee), or unwanted
>> external forces (eg asymmetrical drag from the very upper atmosphere,
>etc).
>>
>> I do agree with you, however, that for an earth-based observer, squint
>> angle changes on an orbit-by-orbit basis.
>>
>>
>> 73,
>>
>>
>> Chris vk6kch
>>
>>
>>
>> At 20:05 01/01/2001 +0100, you wrote:
>> >Hi Chris,
>> >thank you very much for your very clear explanation of ALON/ALAT and
>squint
>> >angle
>> >in the AMSAT BB. I think, there is one aspect that should be mentioned:
>> >Since ALON/ALAT are time dependent, it is of some importance to know to
>what
>> >time the given values of ALON and ALAT refer. The time is not very
>critical,
>> >but one should know the day at least. Unfortunately most times the time
>is
>> >not mentioned in the sources.
>> >
>> >As a test I have calculated (with my own program SatPC32) the
>Squintangels
>> >of AO-40 with AON/ALAT 260/2 1.) as given at Dec.1st and 2.) as given
>at
>> >Dec. 28th 2000. With these different times the squint angles differ about
>5
>> >deg.
>> >Please excuse my bad English.
>> >
>> >Best wishes for then New Year.
>> >
>> >Vy 73s, Erich, DK1TB.
>> >
>> >----- Original Message -----
>> >From: "Chris Hill" <CRH_Telnet@bigpond.com>
>> >To: <T5z4@aol.com>
>> >Cc: <amsat-bb@AMSAT.Org>
>> >Sent: Monday, January 01, 2001 2:32 PM
>> >Subject: Re: [amsat-bb] AO-40 MA, ALON/ALAT and squint angle ??
>> >
>> >
>> >> Hi Steve,
>> >>
>> >> Maybe this way of thinking about it might help...
>> >>
>> >> AO-40 is whizzing around the earth in an elliptical orbit. At the
>moment,
>> >> the satellite is spin-stabilised. This means, that the "front" of the
>> >> satellite is always pointing at a particular fixed point in space,
>> >> regardless of where in the orbit it is.
>> >>
>> >> The whole issue of "ALON/ALAT", or "satellite attitude", or "Bahn
>LAT/LON"
>> >> or "BLAT/BLON" is to describe which particular direction the front of
>the
>> >> satellite is indeed looking at.
>> >>
>> >> Satellite Attitude is measured when the satellite is at apogee
>(furthest
>> >> point from the earth). Think of the Attitude Longitude as equivalent
>to
>> >> Azimuth.
>> >>
>> >> If the front of the satellite (the +z facet) is pointing straight
>towards
>> >> the earth, then ALON = 0 degrees.
>> >>
>> >> If the +z facet is facing straight back to where it has just come from
>> >> (strictly speaking, facing backwards to the tangent of the orbit), then
>> >> ALON = 90 degrees.
>> >>
>> >> If the +z facet is facing directly away from the earth, then ALON = 180
>> >> degrees.
>> >>
>> >> Finally, if the +z facet is facing directly forwards (along the tangent
>> >> line), then ALON = 270 degrees.
>> >>
>> >> Remember, a spin-stabilised satellite (like AO-40 is now) keeps
>pointing
>> >to
>> >> the same point in space... so with a ALON of 270 degrees, whilst the
>+z
>> >> facet is pointing straight forward at apogee, it will be pointing
>straight
>> >> backwards at perigee. AO-40's 400N motor is on the +z facet; by
>having
>> >it
>> >> pointing backwards at perigee, the thrust adds to the satellite's
>orbital
>> >> energy. (This is how the approx 6 minute burn lifted the apogee out to
>> >> 60,000 km).
>> >>
>> >> There's a pretty good diagram showing ALON of 270 degrees at:
>> >> http://www.cstone.net/~w4sm2/software2/ALON.gif
>> >>
>> >>
>> >> As for ALAT:
>> >> Think of ALAT as the satellite's equivalent of Elevation. If ALAT is
>> >> positive, then the +z facet is pointing up, with respect to the orbital
>> >> plane. If ALAT is negative, then the +z facet is pointing down with
>> >> respect to the orbital plane.
>> >>
>> >>
>> >> AO-40's current attitude
>> >> ========================
>> >> I'm using a figure of 260/+2.2. This means that at apogee, the +z
>facet
>> >is
>> >> pointing slightly (2.2 degrees) up, and slightly (10 degrees) to the
>right
>> >> of straight ahead.
>> >> At perigee then, the motor and antennas are pointing almost straight
>> >> backwards, but with a slight (10 degree) angle away from the earth.
>> >>
>> >>
>> >> What's Attitude got to do with Squint Angle?
>> >> ============================================
>> >> Attitude tells us where in space the satellite is pointing. As the
>> >> satellite moves through its orbit, the earth may or may not come
>between
>> >> the satellite and that point in space. Most of the time, the earth is
>> >> _not_ in front of the satellite... it's somewhere off to the side.
>> >> Depending on the satellites attitude, and the geometry between the
>> >> groundstation and the satellite, the squint angle can be calculated.
>> >>
>> >> Most satellite tracking programs that can do this date back to AO-10
>and
>> >> AO-13 days. AO-10 and -13 had the high-gain antennas mounted on the
>> >> "bottom" of the satellite (the -z facet)... but AO-40 has the high-gain
>> >> antennas on the +z facet. This is why you have to modify the published
>> >> ALON/ALAT figures prior to putting them into InstantTrack, etc.
>> >>
>> >> (Figure AO10-1, page C-2 of The Radio Amateur's Satellite Handbook
>seems
>> >to
>> >> be in error here. It shows AO-10's antennas as being on the +z facet).
>> >>
>> >> If the published figures are 260/2.2, then obviously the other end of
>the
>> >> satellite is pointing the other way;
>> >>
>> >> Corrected_ALON = (Published_ALON + 180 degrees) MOD 360.
>> >> eg = (260+180) MOD 360 = 80
>> >>
>> >> Corrected_ALAT = Published_ALAT * -1
>> >> eg = +2.2 * -1 = -2.2
>> >>
>> >> So for Instanttrack, use a figure of -2.2,+80 (IT takes the attitude
>in
>> >> the order of ALAT,ALON).
>> >>
>> >>
>> >>
>> >> I hope this somewhat lengthy explanation has helped a little.
>> >>
>> >>
>> >> Regards,
>> >>
>> >>
>> >> Chris vk6kch
>> >>
>> >>
>> >>
>> >>
>> >>
>> >> At 22:30 31/12/2000 EST, you wrote:
>> >> >In a message dated 12/31/00 4:19:48 PM Eastern Standard Time,
>> >Mike73@aol.com
>> >> >writes:
>> >> >
>> >> ><< A hand drawing or a simple model will help at this
>> >> > point. See the following drawing.
>> >> >
>> >> > www.cstone.net/~w4sm2/software2/ALON.gif >>
>> >> >
>> >> >I wonder what an ALON/ALAT of 0/270 would look like.
>> >> >That might help my understanding of the range of possible
>> >> >attitudes.
>> >> >
>> >> >73, Steve
>> >> >K5PK
>> >> >----
>> >> >Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
>> >> >To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org
>> >> >
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>> >
>>
>> +--------------------------------------------------------------+
>> | Chris Hill mailto:chris.hill@crhtelnet.com.au |
>> | CRH Telnet Pty Ltd A.C.N. 008 922 530 |
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>>
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